1. Field of the Invention
The present invention relates to a contamination monitor for fluids, and more specifically, to a contamination monitor which is capable of measuring fluid contamination at the molecular level in real-time.
2. Description of the Related Art
The cleanliness requirements for the manufacture and operation of sophisticated technical systems are becoming ever more stringent. This is especially true in manufacturing processes involved in microelectronics, high precision optics, as well as in the preparation of systems for flight of spacecraft. It is no longer sufficient just to maintain a certain level of particulate matter in a work environment, as has been the practice for several decades; it is becoming clear that contamination on a molecular level can create serious manufacturing and operational problems.
It is well known that all materials and most activities emanate gases or small aerosols by diffusion and desorption. The term "contamination" refers to a situation where an emitted gas or aerosol impinges and condenses on a surface. Contamination of a "clean" surface typically originates from two main sources: activities or processes in the clean work area and from the materials used in the construction of the article itself (self-induced contamination).
Nonvolatile residue (NVR), sometimes referred to as molecular contamination, on critical surfaces surrounding space structures have been shown to have a dramatic impact on the ability to perform optical measurements from platforms based in space. In such cases, the particulate and NVR contamination originate primarily from pre-launch operations. Molecular deposition on such surfaces affects the thermal balance of a spacecraft scheduled for a long duration mission since the absorbance and emittance of the thermal control panels are adversely affected. Any optical surface (such as windows or mirrors) is degraded by molecular depositions and particulates. Condensed films of contaminants on the order of 10 angstroms thick degrade the efficiency and operation of the optical components. Therefore, a real-time measurement of NVR is required to assure that critical components are fully operational and not subjected to high levels of contaminants during payload processing, storage or on-orbit.
In microelectronics fabrication, the ability to manufacture higher performance and higher density integrated circuits places stringent demands on the physical and chemical properties of materials used. Clearly, as integrated circuit path dimensions shrink to the far sub-micron regime with thinner thicknesses of oxide and metal layers, contamination on a molecular level will have substantial adverse effects. Thus, contamination must be monitored during product processing as well as in process fluids (liquids and gasses) before and during their use.
The currently accepted Semiconductor Equipment Manufactures Institute (SEMI) standard method of measuring NVR in liquids is based on a weight residue technique. The major drawbacks to this method is that it is time consuming, tedious and is not very sensitive. An aliquot of the process fluid is placed in the pre-weighed dish and the sample liquid evaporated to dryness. The weight of the remaining residue is considered NVR and is expressed in parts per million (ppm). The lower limit of detection is around 5 ppm. Other disadvantages of this method is several hundred milliliters of sample are required per assay and the evaporating vapors must be contained and not released into the environment, especially if the process liquid is toxic. A method which requires 1000 times less of the process liquid for each assay and be performed in a few minutes would be advantageous.
Another method to measure NVR in process liquids is based on forming a small aerosol droplets of the process liquid and suspending them in a gas stream to accelerate the evaporation of the liquid. After the liquid has evaporated, any NVR material forms small particles which are detected by an optical counter. The number of particle counts is related to the concentration of NVR material. This technique uses less solvent and is, in general, less time consuming per assay than the weight residue method. However, the instrumentation is extremely sophisticated and known to require highly technical operators to achieve the best performance. The major disadvantage of this technique is that the correlation of NVR deposition on a semiconductor surface from the liquid using particles counts is questionable and not straight forward.
A piezoelectric crystal microbalance has been used for the measurement of mass deposition. Piezoelectric crystals in this category have operated in the bulk-vibration mode wherein the entire body of the crystal is driven electrically into resonance. The piezoelectric crystal operates as a microbalance by the de-tuning of the crystal's resonant frequency when mass is added to its surface.
U.S. Pat. No. 4,561,286 issued to Sekler, et al., discloses such a bulk piezoelectric crystal microbalance. The bulk-vibration method requires the placement of the resonating electrodes on the opposite side of the bulk crystal, wherein the distance between the electrodes, i.e., the thickness of the crystal, defines the resonating frequency of the crystal. Therefore, the resonant frequency of a bulk vibration crystal is inversely proportional to the crystal thickness. The limit of the resonant frequency obtained with a bulk mode crystal is approximately 15 MHz, because a thinner crystal would be too fragile. Since the change in mass detectable by the crystal is proportional to the square of its frequency, the limit of mass resolution in the bulk vibration mode is typically on the order of 10.sup.-8 to 10.sup.-9 g-cm.sup.-2. This level of mass resolution is sufficient to detect contamination at a particle level but is not fine enough to detect contamination at a molecular level.
U.S. Pat. No. 5,476,002, issued to Bowers, et al., which is incorporated herein by reference, discloses a high-sensitivity, ambient, real-time NVR monitor capable of detecting contamination at the molecular level with greater mass sensitivity than previously reported. The greater mass sensitivity was obtained using piezoelectric crystals having a resonant frequency at 200 MHz. The relationship between frequency and mass may be defined mathematically. For example, the change in frequency due to mass addition .DELTA.M, over area A, follows the general form, EQU .DELTA.f=-.alpha.f.sub.o.sup.2 .DELTA.M/A (1)
The coefficient .alpha. depends on the type of crystal and the mode in which its oscillation is excited by the application of an electric field. For a quartz crystal operating in the thickness-shear mode (for an AT-cut bulk crystal), EQU .DELTA.f=-2.2.times.10.sup.-6 f.sub.o.sup.2 .DELTA.M/A (2)
Since the mass sensitivity is a functions of the square of the fundamental oscillating frequency, small increases in the operating frequency give greater performance. However, with the standard bulk crystal operating in the shear mode, as the operating frequency increases, the thinner the crystal must be. A trade-off of mass sensitivity versus crystal ruggedness results in a 10 MHz crystal being the most commonly used as a microbalance as it possesses acceptable mechanical strength with a mass sensitivity around 4.42.times.10.sup.-9 gm/Hz cm.sup.2 (equivalent to 2.3.times.10.sup.8 Hz cm.sup.2 /gm).
Higher resonating frequencies can be achieved by driving the crystal in a surface acoustic mode, wherein the top few atomic layers of the piezoelectric crystals surface are driven in a longitudinal acoustic mode by a series of closely spaced interdigitated electrode transducers which are deposited on the surface of the crystal substrate. The electric field is applied parallel to the surface of the crystal and Rayleigh waves are generated which move along the surface of the crystal. The fundamental frequency of this device is mainly dependent on the configuration of the transducers and not on the thickness of the substrate. Therefore, higher operating frequencies can be achieved without reducing the thickness of the crystal.
Surface acoustic wave (SAW) piezoelectric sensors typically used by researchers in analytical applications are based on SAW delay lines. When two sets of interdigital electrodes are deposited on a piezoelectric crystal at a distance L apart, a standing wave is set up if L=N.lambda., where N is an integer and .lambda. is the wavelength of the surface acoustic wave. The frequency f is equal to v/.lambda. where v is the surface acoustic wave phase velocity. The wavelength is dependent on the spacing, s, between the interdigital electrodes, and is equal to twice the spacing. The bandwidth of the device is determined by the length of each transducer. The transducers serve two main functions, the first is to convert electrical energy from the oscillator circuit into mechanical energy on the surface of the quartz piezoelectric crystal, and vise versa. The second function is to establish the frequency response of the delay line. The Rayleigh surface wave travels in both directions away from the transducer. The surface wave that travels away from the opposite transducer and towards the end of the crystal is lost which results in the delay line being a low Q device. Clearly, a high Q device would provide greater mass-loading dynamic range and possess higher frequency stability.
A factor which limits the lower limit of detection of NVR material in fluids is the probability that 100% of the contaminants present in a sample fluid will deposit or adsorb onto the surface of the SAW device during the time the fluid comes in contact with the sensor. The collection efficiency of a SAW device is influenced by the probability of a given molecular species being adsorbed, the so-called "sticking coefficient." A priori such factors in general are not well-known, but clearly a low collection efficiency will contribute to the time necessary for a reliable measurement. Moreover, for quantitative trace analysis, uncertainty in the collection efficiency can substantially limit the precision of an assay.
Ideally, NVR fluid monitoring should proceed in real-time, and yet a variety of noise sources conspire to increase the time necessary to reach a desired level of accuracy. Clearly, there exists a need for a real-time NVR monitor which can take full advantage of the great sensitivity offered by state-of-the-art SAW devices.
Such a NVR monitor could operate with microliter liquid sample sizes that provide a true measure of NVR deposition. The high sensitivity of such instrument would permit real-time analyses and thus could be used as process control instrument.